1. Field of the Invention
The present embodiment of the invention relates generally relates to the field of electronic paper displays. More particularly, the present embodiment of the invention relates to systems for maintaining DC balance in bi-stable displays using contrast correction and methods for variable-speed page flipping with contrast correction.
2. Description of the Background Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be updated electronically. Some of the desirable properties of paper that this type of display tries to achieve include: low power consumption, flexibility, wide viewing angle, low cost, light weight, high resolution, high contrast and readability indoors and outdoors. Because these types of displays attempt to mimic the characteristics of paper, they are referred to as electronic paper displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper and bi-stable displays.
The reflectance or color of a pixel in a bi-stable display typically changes as voltage is applied. For example, in some bi-stable displays applying a negative voltage to a pixel makes it lighter (higher reflectance) and a positive voltage makes it darker. The higher the voltage and the longer or more times that voltage is applied, the larger the change in reflectance. Electronic paper displays are typically controlled by applying a sequence of voltages to a pixel instead of just a single value like a typical LCD. These sequences of voltages are sometimes called waveforms. The control signals used to drive a pixel depend not only on the optical state the pixel is being driven to, but also on the optical state it is being driven from. Depending on the display technology, other factors may also need to be taken into consideration when choosing the waveform to drive a pixel to a desired color. Such factors can include the temperature of the display, optical state of the pixel prior to the current optical state, dwell time (i.e., the time since the pixel was last driven), and the granularity of control over voltage and duration of pulses that can be applied, and even the particular physical characteristics of a given display panel or batch of panels Failure to take these factors into account can lead to faint remnants of images that have supposedly been erased still being visible, a visual artifact known as ghosting. Some displays also have additional requirements that must be met to avoid damaging the display, such as the requirement that waveforms be DC balanced. EPD controllers will typically use relatively long waveforms (on the order of three-quarters of a second in duration) which use pulses of positive, negative and zero voltages sequenced as to maximize the number of colors or gray levels available and minimize ghosting given the current display environment.
Because of the relatively large amount of time required to update an EPD, the inventor of this patent application has developed a number of methods for transitioning between pages on an EPD in a very fast manner (less than 500 ms per page) referred to as fast page flipping. However, a number of additional problems have arisen when fast page flipping is utilized. These methods typically use short waveforms that are often (though not necessarily exclusively) unipolar, that is, waveforms that consist entirely of voltage pulses of the same sign. Images produced using such waveforms will typically exhibit more ghosting, lower contrast and smaller palette of available gray levels than those produced with longer waveforms, but are advantageous in that they can produce a page transition in a fraction of the time. Short waveforms are especially useful for rapidly displaying a sequence of pages, as one might see when flipping through a book. In this context, speed is more important than image quality. As with longer waveforms, it is important that impulse potential is not allowed to build up across the display or within a given pixel or region, as this can cause long-term damage to the display. Typically this is guaranteed by ensuring that the set of waveforms used to drive the display are DC balanced across the set, as this ensures that the impulse potential applied to any given pixel is bounded. However, this need to maintain DC balance creates new challenges that have not been previously addressed, especially when it entering and leaving fast-page-flip mode and when speeding up during fast page flipping. For example, say a display has been recently driven using a traditional (non-fast) drive scheme that has a maximum impulse potential of 12 pulses ⊕20 ms/pulse ⊕±15 V. To counteract such, this impulse potential would require a waveform of at least 12 pulses in length, because waveforms are duration-limited in the amount of impulse potential they can apply to a given pixel during a transition. The same problem exists when starting at a slower page-flip speed and gradually increasing the page rate, as one might want to do when the user holds down the “next page” button.